This invention relates to transducers used for the measurement of minute displacement, e.g., in the range of several microns or less with precise accuracy. The application also relates to the indirect measurement of other physical parameters, such as temperature and pressure, through the use of a secondary transducer which provides a displacement proportional to the temperature or pressure to be measured. The application relates to the measurement of such physical parameters with extremely high resolution over a relatively small dynamic range, or over a correlated group of dynamic ranges.
The most successful, prior, high resolution displacement transducer known to applicant is a linear variable differential transducer which comprises a transformer having a primary coil and two secondary coils symmetrically spaced on a cylindrical form. A free moving, rod-shaped magnetic core inside the coil assembly provides a path for the magnetic flux linking the coils. When the primary coil is energized by an external source, voltages are induced in the two secondary coils. Typically, these secondary coils are connected as an opposed series circuit so that the two induced voltages, are of opposite polarity. Thus, the net output of the transducer is the difference between these two voltages which is zero when the core is at the center or null position in the transducer. When the core is moved from the null position, the induced voltage in the coil toward which the core is moved increases, while the induced voltage in the opposite coil decreases. This action produces a differential voltage output which varies linearly with changes in core position. The sign of the output voltage changes as the core is moved from one side of null to the other.
Such transducers provide isolation between the electrical circuit and the element whose displacement is being measured, and offer wide temperature range operation, long mechanical lifetime, high sensitivity with low drift and linearity over a broad dynamic range. For example, the sensitivity of such devices provides a minimum measureable displacement as low as 0.02 microns. Other such devices may have a linear range as high as 10 inches.
While such transducers have low susceptibility to hostile chemical and thermal environments, their electromagnetic nature makes them relatively susceptible to electrical and magnetic interference, so that their use in a hostile electromagnetic environment may be prohibited. Furthermore, the dynamic range of a particular transducer, once the transducer has been manufactured, is predetermined, as a trade-off against high resolution, so that a particular transducer may not be used for a variety of applications.
Finally, while the resolution of such transducers is relatively high, certain applications may require measurement of displacements smaller than 0.02 microns and, to date, electromagnetic devices have not been constructed with a higher resolution.